Physical Metallurgy of Modern Creep-Resistant Steel for Steam Power Plants: Microstructure and Phase Transformations

Abstract

The fact that the microstructure of steel depends on its composition and the heat treatment given to it has been heavily exploited in the design of steel for power plant applications. To obtain a steel that can function at the higher temperature where power plants operate without failure for extended life, heat treatment is needed to produce fine and highly stable dispersion of carbides, nitrides, and intermetallic compounds in the microstructure of the material. A significant contribution also comes from solid solution strengthening by substitutional solutes. We review here various types of phases, microstructures, functions, and interacting effects of the various alloying elements in the design of steel for modern power plant application.

Precipitation Characterization and Creep Strength at 600°C for Creep Resistant Cr–Mo Steel

“…The basic Cr-Mo steels are 0.5Cr-0.5Mo, 1Cr-0.5Mo, 2.25Cr-1Mo, 5Cr-0.5Mo, 9Cr-1Mo and 12Cr-1Mo alloyed steels. 4) The use of alloying elements such as, V, W, Ni, Ti, Nb, B and/or N allows to reach the current grades such as the T/P22, T/P23, T/P24, T/P91, T/P92 and VM 12-SHC steels. 1,2) Many of these new grade steels have been applied successfully in industry, but they are also still under development.…”

“…1,2) Many of these new grade steels have been applied successfully in industry, but they are also still under development. This type of steels has been designed [1][2][3][4] to work at temperatures between 350 and 600°C and pressures of about 15-30 MPa for life time of about 250 000 h. Creep resistant steels can support certain stress at specific service temperatures without passing a specified amount of elongation. The maximum stress to rupture at a specific temperature after a specific time, e.g.…”

“…For instance, an engineering design criterion for a power plant could require a minimal stress of 100 MPa for 100 000 h at service temperature. [1][2][3][4] The basic idea is that the vessel keeps original its size and shape during service for up to 20 to 30 years. 4) The creep resistance of a Cr-Mo steel is based on the formation of stable precipitates such as, alloyed carbides in a ferritic, bainitic and/or martensitic matrix phase in the normalized steel condition.…”

“…[1][2][3][4] The basic idea is that the vessel keeps original its size and shape during service for up to 20 to 30 years. 4) The creep resistance of a Cr-Mo steel is based on the formation of stable precipitates such as, alloyed carbides in a ferritic, bainitic and/or martensitic matrix phase in the normalized steel condition. Because of the subsequent tempering treatment, a stable microstructure with precipitates is formed remaining stable at the service temperature for which the steel has been developed.…”

Characterization of creep deformation and rupture behaviour of P92 steel weldment at 600°C

“…In line with the intention to develop the power plant with higher thermal efficiency, having the material that can sustain at high operating temperature and pressure without failure is vital. Instead of relatively cheaper compared to the nickel-based alloy which is ten times costlier [4], P92 steel exhibits fairly high creep strength and ductility, therefore, becomes a potential candidate for such plant. In fact, P92 possess acceptable resistance to the oxidation which makes the material suitable for the long-term application.…”